Oil well servicing mast



7, 1943. J. H. WILSON 2,336,432.

' OIL WELL SERVICING MAST Filed May 24, 1941 '7 Sheets-Sheet 1 Dec. 7, 1943. w Lso 2,336,432

OIL WELL SERVICING MAST Filed May 24, 1941 7 Sheets-Sheet V Dec. 7, 1943. J. H. WILSON 2,336,432

OIL WELL SERVICING MAST I Filed May 24, 1941 7 Sheets-Sheet 3 R1 jTgiE.

I [6 a2 D if fl 7/ E ET Dec. 7, 19.43. J H. WILSON 2,336,432

OIL WELL SERVICING MAST Filed May 2.4, 1941 7 Sheets-Sheet 4 7 v I ll u/ Dec.7,1943. J. H. WILSON 2,336,432 I OIL WELL SERVICING MAST Filed May 24, 1941 7 Sheets-Sheet 5 1943. J. H.'W1LSON 2,336,432

OIL WELL SERVICING MAST Filed May 24, 1941 7 Sheets-Sheet 6 Dec. .7, 1943. J. H. WILSON 2,336,432

OIL WELL SERVICING MAST Filed May 24, 1941 '7 Sheets-Sheet '7 mastshowninFigurel;

Fatented Dec. 7,' 1943 UNITED STATES PATENT OFFICE 2.836.482 Y Y on. WELL sa'avrcmc MAST John Hart Wilson. Wichita Falls, Tex.v Application May 24, 1941, Serial No. 395,102-

Y 10 Claims. (cl. 189-16) This invention relates to improvements in portable masts useful in the servicing of oil well equipment, and is particularly concerned with that type of mast which is formed in telescoping sections, the entire structure being pivotalLv mounted on a motor vehicle, whereby the mast may be readily transported tothe place of use, swung into upright position without removal from the vehicle, and raised to maximum heightby extension of the sections thereof.

It is the general object of the invention to provide a mast of the character described which is so constructed as to facilitate to the utmost the. various operations performed by the use of such masts, such as the withdrawal of well pipe, the vertical racking of sections of pipe during cleaning, repair, or replacement of the well equipment, and the subsequent assembly and lowering of the pipe sections into the well.

It is a further object of the invention to provide an oil well servicing mast of the telescoping type characterized by maximum rigidity and minimum weight, and aifording adequate resistance to stresses imposed thereon during the operations of withdrawing and lowering the pipe.

It is a feature of the invention that the mast. is provided with an elevated platform by means of which the withdrawn pipe sections may be manipulated and racked, the platform being so constructed and arranged as to facilitate collapse and withdrawal thereof into the mast when the latter is retracted for transportation.

A further object of the invention is the provision of novel means for locking the mast sections in extended relation in such manner that the mast will be substantially erect whenloaded'to further the correct application and distribution of stress therein.

Further objects and features of the invention will be apparent from the following description taken in connection with the accompanying drawings, in which Figure 1 is a side elevation of a vehicle mounted mast to which the invention has been applied;

Figures 2 and 3 are partial front and side elevations respectively of the lower section of the mast shown in 1; Figure 4 is a ho substantially on the line [-4 of Figure 2; Figures 5 and 6 are partial front and side elevations respectively of the upper section of the Figures '1, 8, and 9- are horizontal sectional views taken substantially on the lines 1 3-8, and 9 -9 of Figure 6;

ntal sectional view taken I Figure 12 is an enlarged Figure 10 is a side elevation of a portion of the uppermast section illustrating the working and racking platform in extended position;

Figure 11 is a horizontal section of a portion of the mast shown in Figure 10 illustrating the platform in plan;

latch mechanism shown in Figure 10 for retainlng the platform in collapsed 'and withdrawn position; v

Figure 13 isan enlarged fragmentary sectional view taken on the line l3-l3 of Figure 10;

Figure 14 is an enlarged view, partly in section,

, of a portion ofthe structure shown in Figure 10;

Figures 15 and 16 are front and side elevations respectively of thehpper end of the upper section of the mast; I Figure 17,is a front elevation of the upper portion of the lower mast section showing the upper mast section in elevated or extended relation therewith;

- Figure 18 is a side elevation of the structure shown in Figure 17;

Figure 19 is a detailed view of the cooperative locking elements on the upper and lower mast sections; and

Figures 20 and 21'are fragmentary views, partly in section, of details embodied in the structures shown in Figures 1'7 and 18.

In order to facilitate an understanding of the invention, reference will be made to the several embodiments thereof illustrated in'the accompanying drawings and specific language will be employed. It will nevertheless be understood that various further modifications of the devices illustrated herein, such as would fall within the province of those skilled in the artto construct are contemplated as part of the present invention.

Figure 1 illustrates'the general arrangement of the'several elements of the apparatusto which the invention is directed, the mast being shown in full lines in erect and extended position, the

dotted lines indicating the position assumed by the mast when collapsed and lowered for transportation.

' It will be observed that the mast is formed in two sections, a lower section 22 and an upper section 23. The lower section 22' is pivotally mounted at the rear of a vehicle 24 for swinging movement from a generally horizontal to a generally upright, position. Thus a suitable cradle including structural elements 25, 26, and 21 may be securedto the vehicle body so as to extend upwardly and rearwardly therefrom, the cradle detailed view of the.

ter being adapted to engage and withdraw being provided withsuitable means, not shown, in which the lower mast section is trunnioned approximately at the point 28 for swinging movement from the inoperative to the operative position. I

It will be observed by reference to the broken lines in Figure 1, which illustrate the mast when collapsed and swung to horizontal position, that the pivotal axis 28 about which the 'mast is swung is disposed adjacent the upper rather than the lower face of the mast. By so locating this pivotal axis, the mast mayextend further portion of the weight being carriedby the rear vehicle wheels.

The upper section 23 of the mast telescopes within the lower section 22, being elevated or extended by means of a cable 3| which is manipther Windlass 34 on the vehicle is employed to manipulate the cable 35 which passes over the crown sheaves, indicated generally at 36, in the upper portion of the uppermast section 23, and over a suitable block 31 to raise and lower the hook 38 and elevator 39 carried thereby, the latthe well pipe as illustrated in Figure 1.

The general arrangement of the various elements to which reference has just been made is known; the instant invention relates primarily to improvements in the construction of these elements and in the association therewith of further elements hereinafter described. The mast is generally rectangular in horizontal section and in the further description of the invention, that face of the mast which is directed toward the hoisting gear, or toward the right as viewed in Figure 1, will be referred to as the forward face, and the hoisting and stacking operations may be described as conducted at the forward face. The

opposite face, directed toward the forward end,

of the truck,'will be referred to as the rearward face. The remaining faces will be referred to as the side faces of the mast.

Turning now to Figures 2 to 4 inclusive, in

which the structural details of the lower mast section are more particularly illustrated, it will be noted that a framework of structural members is employed, these structural membe'rsbeing ulated by a Windlass 32 on the vehicle 24. A fun soassociated that the forward, rearward and side faces are constituted by a series of truss sub-sections, horizontal sections of the mast being rectangular. The principal load bearing elements are constituted by four upright angle members, one being located at each corner of the structure, .the comer'angles at the forward face beingindicated at "and those at the rearward that, .as

face at 43. These corner angles extend substantiallythe entire length of the lower mast section and constitute laterally supporting and guiding means for the upper mast section. A typical horizontal section is shown in Figure 4, showing the transverselyextending angle members 45 which are secured to the corner angles 42 and 43 at intervals in the length of the mast to form a series of complete rectangles. The forward "and rearward faces of the most are braced by transverse members '45 defining per section within sections of the lower mast section to resist weaving and otherwise .to strengthen the structure. The side faces of the lower mast section are also braced by angle members 41 which extend bebetween and are secured at their opposite ends to.

the corner angles 42 and 43, the members 41 being secured to the members 43 adjacent the each of the subsections of which the mast is composed.

The various structural members of which the mast is composed are preferably welded together, with the exception that the rear face of the lower mast section is bolted to the remainder of the lower section to facilitate assembling and disassembling of the upper and lower mast sections. Thus the rear face of the lower section may be prefabricated by the welding of the various structural elements, and angle brackets'5i are welded to the rear corner angles 43 at intervals in the length of the latter. Bolted to each of the angle brackets 5| is a second angle bracket 51, and the adjacent structural elements 45 and 41 are welded to the bracket 51. This arrangement permits thelocation of welding jigs within the lower section to support the various structural elements during the welding operation, the jigsbeing withdrawn-by unbolting and removing the rear face of the lower mast section on completion of the welding operation. 'The upper section, hereinafter more particularly described, may then be prefabricated, introduced in the lower section, and the rear face of the lower section again bolted in position. The elements just described are shown in some detail in Figure 4.

The load transmitted downwardly througheach of the corner angles 42 is carried by pedestals 49, th latter being braced to the rearward corner angles 43 by a series of diagonally extendingmembersSO. A pulley 52, carried principally by members 53 and 54, is provided for the cables 29 which serve to swing the mast from inoperative to operative positions about its horizontal axis of support on the vehicle.

The upper end of the lower mast section 'is shown in Figures 17 and 18 in association with the lower end of the upper mast section. Pulleys 55 and 56, suitably are utilized for raising the upthe lower section in a manner hereinafter more particularly described, and means are provided for retaining the upper section inraised position, as hereinafter explained. g now to Figures 5 to 9 inclusive, which the upper mast section, it will be noted in the case 'of the lower mast section, horizontal sections are generally rectangular in outlin with the important exception that over a substantial portion of the length thereof the forillustrate ward face is in effect recessed, the side faces extending forwardly beyond the rectangular outline so as to afford clearance for the hoisting gear and to provide-for. the reception of the platform in 'the upper mast section when the platform is folded. As in the case of the lower mast section, the vertical load is carried 'primsrly on corner angle members, the forward corner angles being indicated at 53 and the rearward corner angles at 53.. The forward andrearward corner angles at opposite sides of the mast section are connected by angle members 89, and transverse angle members 8| connect the rearward corner angles 59. Extending between and secured to the members 60, are angle members 82; a V-shaped brace constituted by angle members 63 extends between th central portion of each member BI and the forward corner angles 58, being secured also to the members 62. Thus the members 60 to 68 inclusive define generally horiontal planes at intervals in the length of the upper mast section, typical horizontal sections being represented in Figures '7, 8, and 9, the upper mast section being thereby divided into sub-sections or short lengths.

. The forward, rearward, and side faces of each sub-section are reinforced by a truss structure similar to that employed in the lower mast secsection need not be provided with the recessedforward face hereinbefore referred to, and the lower ends of the corner angles 58 and 59 are united by a rectangular members 10 of rectangular or box-section. In the three lower sub-sections of the upper mast section the corner angles are uniformly spaced, the dimensions being such that these comer angles may be received with a. slight clearance within the comer angles 42' and 48 of the lower mast section for sliding and telescoping movement therein, suitable wear pads ll being preferably secured to 58 and 59.- Beginning tion of the upper mast section, the rearward corner angles 59 are deflected toward the forward corner angles 58 so that the upper portion of the upper mast section decreases in transverse sectional area in an upward direction, corner angles 58 and 59 meeting at the extreme upper end of the mast. The uppermost section of the mast is reinforced by a suitable rearwardly extending truss structure, as shown in Figures 15 and 16, and the crown sheaves 36 are mounted on a transversely extending channel member 13 spanning the corner angles at their point of juncture at each side of the mast, so that the compression load applied during lifting oper ations by the crown sheaves is transmitted directly downward through the forward corner angles 58 to the box member 10 at the lower end thereof. It will be observed that the upper mast section is enlarged at its upper end by the truss structure shown in Figures 15 and 16.- The dimensions of this structure are preferablysuch that while the mast sections may be completely telescoped, they will fit together with only slight clearanc when in telescoped relation, the rearwardly extending truss structure of the upper section supporting the latter when the mast is collapsed for transportation.

While it is preferred, in thefabrication of the mast sections, that the various structural members shall be welded at their points of connection, so that each of the two mast sections shall constitute in effect an integral rigid framework, riveting or other means of securing the structure comprising the outer faces -of the corner angleswith the fourth sub-secstructural elements together may nevertheless be resorted to under some circumstances. Structural members of. a diiferent type from those specifically described may be employed within the scope of the invention, the foregoing detailed description being merely illustrative of one form of structure characterized by extreme rigidity and light weight.-

Turning again to Figures 1'7 to 19 inclusive, it will be observed that the cable 8|; by means of which the upper section is elevated, passes over the pulleys and 58 secured to the upper end of the lower section, beneath pulleys 15 and I8 secured to the lower end of the upper section adjacent the rearward face thereof, and is returned upwardly and fastened to the upper end of the lower section-in any convenient manner.

Secured beneath the box member 10 to which the compression load of the upper section is transmitted are shoes 80, these shoes being disposed at opposite sides adjacent the forward face of the mast. Journalled in suitable bearings at each side of the lower mast section are rock shafts 8|, each of these rock shafts carrying an'arm82 which is arranged to engage beneath the adjacent shoe 80 to afford support for the upper mast section in its elevated position. Arms 88 on the rock shafts 81 are connected by alink 84, adjustable as to lengt and an arm 85 on one of the rock shafts l is connected by a similarly adjustable link 88 to a lever 81 pivoted on the lower mast section, releasable pawl and ratchet'mechanism being associated with the lever to retain the same against unintentional displacement. when the lever 81 is rocked in a downward direction, each of the arms 82 is brought beneath the; associated shoe 89 and occupies the position illustrated in Figure 19.. The engaging surfaceslof the arms 82 and shoes 80 are preferably cylindrical segments of which the axis, indicated at 88, is slightly offset from the axis of the associated rock shaft 8|, so that when the arms are partially withdrawn from the shoes, as shown in dotted lines in Figure 19, ample clearance is afforded between the cooperating surfaces. Thus the arms 82 may readily be moved into position beneath the shoes 89, but on completion of this'movement, which is limited by engagement of a pro- :lection 89 on the upper end of each arm 82 with the associated shoe 88, the cooperating surfaces of the arms .82 and shoes 88 will be tightly engaged.

Referring now to Figure 20, illustrating on a larger scale certain of the structure shown in Figure 18, it will be noted that bracket means indicated generally at 90, mounted on and adjacent the upper extremity of each corner angle 82 of the lower mast section, affords a support for a threaded element 9!. Adjustably mounted on the threaded element 91 is a shoe 92 having a downwardly-directed bevelled surface 98 coacting with a correspondingly bevelled surface on a shoe 98 secured to the adjacent corner angle of the upper mast section. It will be appreciated that as the upper mast sec= I tion approachesthe upper limit of its movement,

these coacting bevelled surfaces will engage and exert a rearward thrust on the upper mast section. By slightly adjustment of the position of the shoe 92 longitudinally o the mast, thisforward wedging action may be completed so as toforce the corner angles 59 of the upper mast section against the upper extremity of the rearward corner angles 43 of the lower mast section when the, upper mast section occupies such a position that the coacting surfaces of the arms 82 and shoes are in full engagement as shown in Figure 19, and the arms thus serve to maintain this rearward wedging action on the upper mast section.

In Figure 17 is shown a spindle H0 suitably mounted on the lower mast section and carrying a bevel gear I H. A coacting bevel gear 242 carries a sprocket over which is passed a chain I 45. The chain I45 engages sprockets I 46, each sprocket being mounted on a spindle It'd having threaded engagement in a boss I48, as,

shown more particularly in Figure 21. The bosses I 48 are mounted respectively on the corner angles 43 of the lower mast section, and the spindles I41extend through these corner angles and engage at their free ends I50 with the adjacent faces of the corner angles 59 of the upper section. Thus when the spindle I60 is rotated manually, the sprocket chain I45 rotates the spindles I" in such a direction that the latter engage and thrust forwardly the lower ends of the corner angles 58 of .the upper mast section.

It will thus be seen that I have provided means at the upper end of the lower mastsection for wedgingthe adjacent portion of the upper mast section rearwardly, and means at the lower end of the upper mast section for forcing the latter forwardly, whereby the upper mast section may be tilted rearwardly when free from load. The extent of tilting is slight, but is preferably sufllcient to ensure that when loaded to approximately rated capacity, the forward corner angles of the mast will be substantially straight, and the load will be transmitted directly downward from the crown sheaves 36- through the substantially aligned and straight corner angles 58 and 42 to the pedestals 49 and thence to jacks or other suitable objects placed beneath the pedestals. In other words, by meansof the mechanism just described, it is possible to tilt the upper section on the lower section so as to bow the mast forwardly. It will be appreciated that when the load applied to the mast, the structural members at the forward side of the mast will be subjected to the greatest compression, with the result that the central portion of the mast will be deflected rearwardly. By initially tilting rearwardly or bowing the mast forwardly, as herelnbefore set forth, the effect of the load on the mast may be substantially counteracted, and the load carrying capacity is accordingly substantially increased.

in Figures to 14 inclusive, is shown the collapsible platform which facilitates manipulating and racking of the withdrawn pipe sections. When viewed in plan, as shown in Figure 11, the platform is generally U-shaped, having the legs of the U directed toward the mast, so as to provide an opening through which the lifting gear may be raised and lowered and through which the pipes may extend when elevated, the pipes being retained when stacked by the surrounding platform. The platform is supported principally by a U-shaped box frame indicated at 95, the forward ends of the frame being pivoted at 96 to the mast. Supported on the frame 95 are the lateral rectangular aprons 91, a rearward rectangu-' lar apron 98, and two generally triangular aprons 99, these aprons forming, when extended laterally, a catwalk about the generally rectangular opening defined by the frame 95. As shown in tar-balanced latch I25 pivoted on an arm I 26' 'spective sockets. Similar sockets supporting framework of angle sections IOI carrying a. sheet metal member I02 forming the floor ofthe apron. Each rectangular apron is separately pivoted to the supporting box frame so as to permit the same to swing upwardly and inwardly, so that when folded these aprons occupy a position within the confines of the box frame. Inwardly directed abutments I 03 are pro vided on the latter to support the apron 98 in its inwardly folded position, and the aprons 91 thereon. The triangular aprons 99 may be hingedly supported on adjacent structures but are preferably detachably mounted for removal prior to folding of the rectangular aprons.

The platform is surrounded by a railing, serving the function of protecting the operator. Thus a stanchion I 05 is pivotally connected to each outer corner of each of the rectangular aprons and also to the central portion of each of the aprons 91, each stanchion being formed in two sections which are pivoted together as indicated at I06 and retained in aligned position by a sleeve i0? which is arranged to slide over the adjacent ends of the stanchion sections. The lower sections of the stanchions associated with each of the aprons are reinforced by a tubular bar I 09 extending therebetween and secured thereto, and a similar tubular bar-unites the upper sections of these stanchions. ,The stanchions I05 adjacent the mast are provided with sockets III, and corresponding sockets II2 are provided on the mast, so that these stanchions may be locked in upright position with relation to the mast by inserting the angularly disposed ends of a rod H4 in the re- I I6 are provided on the stanchions I05 at the rear outer corners of the aprons 91 and 08, so thatthese stanchions may similarly be locked together by means of bars II 1. Cables II8 are secured to the box frame 95 at the juncture of the legs thereof with the base, indicated at I I9, these cables passing over pulleys I20 supported on the upper mast section adjacent the upper end of the latter, and thence extending downwardly to the vehicle or some other point for convenient manipulation to raise and lower the platform.

It will be apparent from the foregoing description that the railing around the platform may be collapsed and swung inwardly on the associated aprons and that the latter may then be swung upwardly and inwardly into a position within the confines of the supporting frame 95, as hereinbefore suggested. When so collapsed, each platform and its associated railing will appear substantially as shown in dotted lines in Figure 13. The removal of the rods H4 permits the lowering of the collapsed platform which.

swings about the point of pivotal connection 96 with the mast to a position within the confines of the mast and adjacent the recessed rearward face thereof, the dotted line position of the with drawn platform being shown more particularly in Figure 12. This figure also discloses a. method of retaining the platform in collapsed and withdrawn position, consisting essentially of a. counwhich is in turn pivoted to a transverse angle member I21 forming part of the upper mast section structure. To the upper end of the arm I26 is pivoted a member I28 which extends through an apertured element I29 mounted on an angle member 68 of the upper mast section. The member I28 is threaded at its outer end to receive a nut [30 provided with a handle I3! by which the same can conveniently be turned. A coil spring I32 is connected between the latch I25 shown in Figure 12 automatically occurs when the platform is swung downwardly against the mast.

If the nut I30 be turned downward after latching engagement occurs, the arm I26 will be displaced to the left to increase the friction between the latching surfaces so as to prevent unintentional release of the platform. The nut l30'further affords a means whereby theproper position of the toe of the latch I25 may be established so as to ensure that the platform will be locked in a position clear of the lower mast section when the mast sections are telescoped.

In the use of the apparatus, an operator standing on the extended platform may manipulate the hoisting gear so as to stack the pipe in sections consisting of two standard pipe lengths within the confines of the platform as shown in Figure 1. The recessing of the forward face of the upper section of the mast affords adequate clearance for upper structurein raised position, and means for tilting said upper structure slightly rearward relative to said lower structure whereby said mast is bowed slightly forwardly when not under load, and is substantially straight when under normal load. v

4. A portable mast for use oil drilling operations comprising a lowerstruc- 'tdre; an up structure mounted in telescoping relation with said lower structure, means mounted adjacent the upperend. of said lower structure for supporting said upper structure in raised position, and means for tilting said upper structure slightly. rearward relative to said lower structure whereby said mast isbowed slightly forwardly when not under load, and is substantially straight when under normal load.

5. A portable mast for use in connection with oil drilling operations comprising a lower structure, an upper structure mounted in telescoping relation with said lower structure, means mounted adjacent the upper end of said lower structure for supporting said upper structure in raised position, and means for tilting said upper the hoisting gear without appreciable reduction of strength, beside affording space for the stowing of the collapsible platform. The corner angles. of the two sections, and particularly the forward corner angles, aiford adequate and direct support for the load and also serve as guide members by means of which the two mast sections ar directed during the telescoping movement thereof.

Having thus described the invention, what is claimed as new and desired to be secured by Letters Patent is:

l. A portable mast adaptedto be pivotally mounted on a mobile base for use in connection with oil drilling operations, comprising a lower structure of rectangular section and substantially uniform in transverse dimensions throughout its length, an upper structure mounted in telescoping relation with said lower structure,

said upper structure being of generally rectangular section, the forward face of said upper structure being spaced rearwardly of the forward edges of the adjacent side faces to provide a recessed front, and a collapsible working platform connected to said upper structure in such manner as to be movable to a generally horizontal position when said mastis in raised position and to be folded into the recessed front of said upper. structure when not in use.

22. In combination with the construction defined in claim 1, latch-means associated with said upper structure for engaging and retaining said platform in folded position, and means for locking said latch means against accidentaldisengagement. 7

3. A portable mast adapted to be pivotally mounted on a mobile base for use in connection with oil drillingoperations, comprising a lower structure of rectangular section and substantially uniform in transverse dimensions throughout itslength' an upper structure mounted in telescoping relation with said lower structure, said upper structure being of generally rectangular section, the forward'face of said upper structure being spaced rearwardly of the forward edges of the adjacent side faces to provide a recessed front, means mounted adjacent the upper end of said lower structure for supporting said structure slightly rearward relative to said lower structure whereby said mast is bowed slightly forwardly when not under load, and is substantially straight when under normal load, said last named means including cooperating wedges carried by said lower and upper structures and engaged as said upper structure is raised.

6. A. portable mast adapted to be pivotally mounted on a mobile base for use in connection with drilling operations, said mast comprising upper and lowertelescoping sections, and means acting between said sections to lock the upper section in extended position, said means including at least one arm pivoted on said lower section and a shoe rigid with said upper section,

said arm and shoe being formed to provide coopcooperating surfaces exerting a rearward thrust on said upper mast section when the latter is extended to tilt the same with respect to the lower mast section.

8. A portable mast adaptedjto be pivotally mounted on a mobile base for use in connection with drilling operations, said mast comprising upper and lower telescoping sections, means adjacent' the upper end of the upper section for supporting hoisting gear at the forward side of said mast, means acting between said sections to lock the upper section in extended positionwith the two sections in telescoped relation over a substantial portion of the length thereof, means acting between said sections adjacent the upper end of the lower section for exerting a rearward thrust on the upper section, and means acting between said sections adjacent the lower end of the upper section for exerting a forward thrust on said upper section, whereby said upper section is tilted rearwardly when unloaded to compensate for forward bending of the mast on in connection with application of a normal load to the hoisting gear.

9. A portable mast adapted to be pivotally mounted on a mobilebase for use in connection with drilling operations, said mast comprising upper and lower telescoping sections, means ad- J'acent the upper and of the upper section for supporting hoisting gear'at the forward side of said mast, means acting between said sections to lock the upper section in extended position with the two sections in telescoped relation over a substantial portion of the length thereof, and means applying thrust between said sections in opposite directions at vertically spaced points for tilting the upper section rearwardly when unloaded to compensate for forward bending of the mast on application of a normal load to the hoisting gear.

JOHN HART VVIISON. 

